In medical X-ray diagnostics both sharp contrast and high resolution are aimed at. At the same time, attempts are made to minimize the dosage of radiation received by the patient.
In X-ray photographing, in order to guarantee successful photographing irrespective of the quality of the object to be photographed and of the depicting apparatuses, it is known in prior art to provide the X-ray apparatuses with an automatic exposure system, which takes care of correct exposure of the X-ray film and thereby eliminates the necessity of renewed photographing.
In the prior-art automatic exposure systems, for example, an ionization chamber or a semiconductor has been used as the detector for the radiation passing through the film, the detector being placed behind the object to be photographed and behind the depicting means to measure the quantity of radiation arriving at the film. The signal, which is proportional to the radiation and suitably amplified, is linearized if necessary and thereupon integrated in relation to time. The result of integration is compared with the set value corresponding to the desired darkening of the film, and upon reaching of the value the radiation is switched off. In more advanced solutions, the creep of the base values of the detector and of the signal-processing stages have been compensated for by automatic resetting to zero, which is always carried out in between the photographings.
In relation to the present invention, reference is made to the FI Patent Applications Nos. 882490 and 894903 (applicant Automed Oy). The method and the device in accordance with the present invention can be applied particularly well expressly to the methods and devices described in the FI applications, in combination with which the present invention provides certain advantages of synergism.
In respect of the prior art most closely related to the present invention, reference is made to the FI Pat. Appl. No. 853317, to the DE Published Patent Application No. 36 41 992, and to the U.S. Pat. Nos. 3,974,385 and 4,763,343.
From the patent publications mentioned above, a method is known for regulating the exposure in a mammographic apparatus so that the radiation passing through the breast to be photographed is detected by means of one detector. In the U.S. Pat. No. 4,763,343 it is suggested that two detectors be employed, one of which measures the radiation passing through the tissue to be studied, whereas the other detector measures the radiation by-passing the tissue to be studied, the latter detector being used as a calibration signal for the regulation system. In the FI application 853317 it is suggested as a variation that, by means of one detector-in the automatic exposure system, a number of observations of the radiation passing through the organism and corrections of the spectrum are carried out one after the other during the photographing.
However, the prior-art solutions involve certain drawbacks, such as factors hampering the operation and the calibration, the object of the present invention being to eliminate the drawbacks.
One drawback present in prior-art solutions, in particular in mammographic photographing, is the fact that there is only one detector for measurement of exposure, which must be displaced to different locations depending on the size of the object to be photographed and on the projection of photography in order that correct exposure could be obtained.
As a rule, the displacement of the detector has been carried out manually, in which case the location of the detector and, thus, also the correct exposure must be taken care of by the operator of the mammographic apparatus. It can be justly considered that the reason for such a solution is the difficulty of calibration of the detector. If there are several detectors in stead of one, they all must be calibrated separately.
In the prior-art solutions, the creep of the base values of the signal-processing stages and of the detector has been compensated for by means of automatic resetting to zero, but possible alteration of the amplification has not been corrected. Since amplification of the signal has an essential importance for successful measurement and since the signal given by the detectors in relation to the amount of X-ray radiation varies considerably, in the prior-art solutions the calibration has always been carried out by adjusting the amplification by means of a regulation member manually, by test photography, and by measuring the darkening of the film. In such a case, the operation of regulation is difficult and time-consuming, for, as a rule, the regulation members are placed in the area of effect of radiation, which requires that the regulating person must always move to protection for the time of test irradiations. Also, such regulation members as well as the detector itself have a tendency of alteration in the long run, whereby the measurement result obtained no longer corresponds to the original setting and requires renewed calibration by a person skilled in the art.
Since the detector signal, e.g., in the case of semiconductor detectors does not correlate directly with the darkening of the film, but depends on the kV-value used for the X-ray tube, on the film type and amplification plates, on the cassette type, grid, filter type, and on the exposure time as the film sensitivity is lowered in accordance with the reciprocity law as the time becomes longer, a detector signal integrated in relation to time cannot be used directly for switching-off the radiation when the signal reaches the set value.
Thus, in more advanced prior-art automatic exposure systems, to improve the result, various value tables, stored in the memory of a microprocessor, have been used, in which tables, depending, for example, on the kV-value, filtration, and on type of film/amplification plate, the correct table value has been taken into use for switching-off the radiation. The measurement of such tables is highly laborious, they take an abundance of memory and program space, and their alteration is equally difficult as the creation of a new table. Thus, for the arrangement of measurement in accordance with the present invention, a novel mode of signal processing has been developed, which permits quick adapting of new types of films/amplification plates or of new filter materials even by the operator, and in the present invention just a few storage locations of memory space are required per each type combination.
The object of the present invention is to provide such a method and a device for automatic exposure in mammographic photography that different objects of photography can be photographed optimally with a sufficiently low dosage of radiation.
A further object of the invention is to provide a method and a device for automatic exposure in mammography, the method and device having versatile properties and being advantageous to accomplish.